Bone anchored hearing aids are essential for the rehabilitation of patients suffering from hearing losses for which traditional hearing aids are insufficient. The most common type of such devices consists of an external hearing aid with a vibrating transducer which, through a coupling, is connected to a skin-penetrating abutment which has an interconnection to a screw shaped fixture anchored in the skull bone. The fixture is usually made of titanium and is usually designed with a flange to prevent the fixture from being pushed through the skull bone in case of a sudden accidental impact. The surgery for anchoring the fixture and the abutment is often done in one stage where the fixture and the abutment are inserted at the same time. However, for some patients it is necessary to do a two stage surgical procedure where the fixture is inserted in the bone and then left to integrate with the bone for 3 months before the skin penetration is done and the abutment is attached. Since this kind of concept includes a permanent skin penetration a significant number of these patients have problems with skin infections due to bacterial growth in the area.
The abutment can either be mounted on the fixture with a small connection screw going through the center of the abutment and into a threaded hole in the fixture. Alternatively, the abutment and fixture can be manufactured in one single piece of material.
The connection between the fixture and the abutment is of critical importance for the clinical function of this kind of product system. If the abutment and fixture is integrated and manufactured from one piece of material, it is not possible to separate the abutment portion from the fixture portion without machining the material with precision drills etc. Such metal machining procedures are not desirable in a clinical situation. It is important to have the possibility to remove the abutment in case it has been damaged and need to be changed or if the skin-penetrating abutment should just be removed from the patient. If the fixture and the abutment are manufactured in one piece it is also not possible to optimize the material choice for the fixture and the abutment separately. Another drawback with conventional designs is that it is not possible to do the surgery in a two stage surgical procedure that is necessary for some patient groups.
The alternative to the integrated design is to have a separate abutment mounted on the fixture with a small connection screw going through the center of the abutment and into a threaded hole in the fixture. The abutment rests on a planar surface on the fixture and is kept in place only by the connection screw. Since the surfaces are usually normal machined surfaces there will always be small gaps in the interface between the abutment and the fixture. In such gaps bacteria can grow and also be transported from the inside of the abutment to the skin penetration area, which may result in an increased risk for infections in the skin penetrating area. A common problem with conventional designs is also that the connection screw and the abutment sometimes comes loose which causes both poor sound quality and an increased risk for skin infections. The connection screw is a quite expensive component since it is often made of a gold alloy.
Another current problem causing skin irritation is the fact that often the skin around the abutment, which has been thinned down to a skin thickness of around 1 mm during surgery, grows with time in the lateral direction around the abutment. For the current designs there is nothing that hinders the skin from growing thicker around the abutment as time goes by. Thick skin around the abutment significantly increases the risk for skin infections. There is a need for a hearing aid system that does not have the above-outlined drawbacks.